1. Field of the Invention
The present invention relates generally to an apparatus and method for controlling forward link power in a mobile communication system, and in particular, to an apparatus and method for controlling forward link power in a high-speed packet transmitting mobile communication system.
2. Description of the Related Art
In general, a high-speed packet transmitting mobile communication system supports data channels only or both data channels and voice channels. The former system type is an IMT-2000 (International Mobile Telecommunication-2000) 1×EV-DO (Evolution-Data Only) and the latter system type is an IMT-2000 1×EV-DV (Evolution-Data and Voice).
In these high-speed packet transmitting systems, a plurality of users share the same packet data channel in TDM (Time Division Multiplexing), for high-speed data transmission. A base station (BS) receives feedback information concerning a forward channel status from a mobile station (MS). If the forward channel status is good, the BS transmits data using a high-order modulation scheme such as 8-PSK (Phase Shift Keying), 16-QAM (Quadrature Amplitude Modulation), or 64-QAM. Since a forward packet data channel is shared in TDM among a plurality of users, the BS assigns all available power to a single user for the time assigned to the user.
In view of the burstiness of packet data, there are periods where packet data is not being transmitted. In other words, packet data is discontinuously transmitted. Therefore, when a small number of users share a packet data channel or packet data delay results in repeated discontinuous packet data transmission, it is possible that an automatic gain controller (AGC) used for maintaining a constant received signal power cannot keep up with the power change of the packet data channel in the MS. As a result, the reception quality of the packet data channel is degraded if a high-order modulation scheme such as 16-QAM is used. This will be described with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram of a conventional AGC for maintaining a constant received signal power and FIG. 2 is a timing diagram illustrating transmission power variations in a transmitter and received signal power variations in a receiver when a packet data channel is discontinuously transmitted.
Referring to FIG. 1, a gain control amplifier (GCA) 10 amplifies a signal S(t) received through an antenna (not shown) according to a feedback signal. The amplified signal is output and simultaneously provided to an accumulator 20. The accumulator 20 accumulates the received signal for a predetermined time. An adder 30 adds a negative-signed reference signal voltage—AIM_AMP to the output of the accumulator 20. That is, the adder 30 obtains the difference between the output of the accumulator and the reference signal voltage-AIM—AMP. A feedback loop filter 40 having a predetermined bandwidth filters the difference signal and the GCA 10 amplifies an input signal according to the filtered difference signal.
Referring to FIG. 2, reference symbol Ior denotes BS transmission power, reference symbol Îo denotes the power level of a received signal controlled by the AGC 10, and reference symbol Îor denotes BS transmission power measured at the receiver according to the change of the received signal power level Îo. The BS transmission power is Pmax during a packet transmission period, while it is reduced to Pnormal during a non-packet transmission period. On the other hand, the received signal power Îo is not maintained constant in the AGC 10 due to a great change in the transmission power at a point A where packet data transmission starts or at a point B where the packet data transmission is discontinued.